1. Field of the Invention
The present invention relates to a semiconductor substrate exposure method of a semiconductor substrate and more particularly, to a semiconductor substrate exposure method that is effective in an optical exposure process using a step-and-repeat manner exposure technique using ultraviolet (UV) light.
2. Description of the Prior Art
To form integrated circuit devices (ICs) on a semiconductor substrate or wafer, generally, a reduction step-and-repeat projection exposure system has been used to transfer patterns of geometrical shapes on an exposure mask or reticle to the wafer.
Specifically, a positive photoresist film is formed on the entire surface of the wafer and then, a small image field or chip site of the film is exposed to a projection of UV light at one time. After the exposure of the image field, the wafer is moved to the next image field and the process is repeated. Thus, the photoresist film is partitioned into a plurality of image fields by step-and-repeat projection of UV lights.
In this specification, the region where the patterns are transferred, in other words, where the plurality of chip sites or image fields are formed, is termed the "image formation region". On the other hand, no patterns are transferred to the periphery of the wafer. The region where no patterns are transferred is termed the "untransferred region".
Typically, the untransferred region of the photoresist film is removed from the wafer during a development process subsequent to this step-and-repeat projection process. Conventionally, this process has been performed by the following steps.
As shown in FIG. 1, first, a reticle with specified patterns for at least one chip site is loaded onto a reduction step-and-repeat projection exposure system (step 101). Then, the position of the reticle is adjusted so as to be aligned with the reference position of the system (step 102). A semiconductor wafer with a positive photoresist film thereon is placed on a wafer stage of the system and is adjusted so as to be aligned with the reference position (step 103).
A plurality of optical exposure processed are performed in a step-and-repeat manner (step 104) for the respective image fields or chip sites of the photoresist film. Thus, the patterns on the reticle are transferred to the respective image fields.
The patterned reticle is removed and then, a blank reticle is loaded onto the exposure system (steps 105 and 106). The blank reticle has no pattern thereon and as a result, the exposing UV light passing through the reticle will have a uniform intensity within the entire cross section perpendicular to its optical axis.
The reticle alignment and wafer alignment processes are performed for the blank reticle (steps 107 and 108). Then, a plurality of optical exposure processed are performed in a step-and-repeat manner for the untransferred region of the photoresist film (step 109). Thus, the entire untransferred region is exposed to the UV light.
Finally, the photoresist film is developed (step 110). Through this development process, the patterns on the reticle are transferred to the respective image fields in the image formation region of the film. At the same time, the untransferred region of the film is entirely removed.
With the conventional exposure method described as above, use of the blank reticle necessitates reticle replacement time (approximately five minutes) and wafer alignment time (approximately one minute for one wafer) thereby resulting in the problem of decrease exposure system throughput.
The use of the blank reticle also causes another problem in that IC fabrication cost is increased.
To solve these problems, another conventional exposure method was developed, in which a patterned area for pattern transfer and a blank area for untransferred-area exposure are provided on the same reticle. This method was disclosed in the Japanese Non-Examined Patent Publication No 2-62541 published in Mar. 1990.
In this method, when the patterns on the reticle are transferred to the pattern formation region of the photoresist film, the step-and-repeat exposure steps are performed using the patterned area. The blank area if the reticle is used for the step-and-repeat exposure steps of the untransferred region of the film.
However, this conventional method requires that the reticle be capable of pattern formation of at least two IC chips, and therefore, this method cannot be used if the reticle is only capable of pattern formation for one chip.
Further, the necessity of the blank area decreases the number of the chips that can be exposed during each step or shot of the step-and-repeat exposure process. This also leads to reduce exposure system throughput.
Specifically, for example, when the reticle has two pattern areas for two IC chips, each exposure shot of the step-and-repeat exposure process can irradiate two image fields at a time, and therefore, the exposure shot needs to be repeated at 50 times for 100 chips. However, when the reticle has pattern and blank areas, each exposure shot irradiates only one image field, and as a result, the exposure step needs to be repeated 100 times for 100 chips.